High Intensity Discharge (HID) lamps have found widespread application in a number of commercial and industrial settings not only because of their obvious quality of providing a high intensity light source but also because such high intensity light output has been achieved with a high degree of luminous efficacy measured in terms of lumens per watt (LPW) and further because the color rendering capabilities of such lamps have proven to be quite acceptable. In typical HID lamp, a medium to high pressure ionizable gas is caused to emit visible wavelength radiation by the passage of current through a pair of electrodes which are disposed within the arc tube containing the gas. In response to the fact that such electrodes may be subject to energy loss, evaporation and chemical attack by the gas constituents of the arc tube, recent design efforts have been directed along the lines of removing the electrodes altogether.
In a typical electrodeless HID lamp which is inductively driven, the configuration will include a coil disposed around a bare quartz arc chamber containing the ionizable gas. The plasma within the arc chamber will be excited by a high frequency inductively coupled magnetic field. Of course it can be appreciated that an electrodeless HID lamp can be driven by a capacitively coupled electric field as well. An example of an electrodeless HID lamp of the inductively coupled design can be found in U.S. Pat. No. 4,972,120 issued to Witting on Nov. 20, 1990 and assigned to the same assignee as the present invention. As described in this cited reference, the arc tube is surrounded by a coil and is supported by means of a rod member.
The lack of an outer jacket relative to the bare arc tube described in the above cited reference can have a number of disadvantages to the practical, commercial application of this technology to an HID lamp offered in the marketplace. For instance, a bare arc tube may be easily susceptible to actual physical damage particularly in view of the fact that the HID lamp must ultimately be placed in a lighting fixture in order to have a practical use. Additionally, it is known that, with a bare arc tube typically constructed of quartz, hydrogen, oxygen and/or water vapor from the ambient air may diffuse into the arc chamber thereby affecting the mixture of gas constituents within the arc tube. Then there is a related problem that the bare quartz arc tube may be subject to surface contamination which can cause the devitrification of the quartz material that forms the arc chamber. One way to avoid the above discussed problems of a bare arc chamber used for an electrodeless HID lamp is the provision of an outer jacket which surrounds the arc chamber. An example of an electrodeless HID lamp having an outer jacket in addition to the arc chamber can be found in U.S. Pat. No. 4,810,938 issued to Johnson et al. on Mar. 7, 1989 and assigned to the same assignee as the present invention. Though this earlier patent does disclose the use of an outer jacket in surrounding relation to the arc chamber, the primary emphasis of this patent is toward the combination of arc tube materials which result in the improved efficacy and color rendition performance characteristics of the lamp. What discussion there is in this reference relating to the structure of the lamp is directed to the desirability of improving the shape of the arc tube such that, in cooperation with the designated fill constituency, the arc tube operates essentially as an isothermal device; that is, the arc tube will not experience the same thermal losses found in a typical electroded HID lamp. To further insure the optimum thermal performance of this electrodeless HID lamp, this patent discloses the use of a thermal energy barrier means between the arc tube and the outer jacket. This reference does not discuss any manner of applying the HID lamp to a lighting fixture nor an alternative approach to thermal management of the heat generated within the outer jacket by the discharge occurring within the arc tube upon excitation of the gases contained therein.
It would be advantageous if an electrodeless HID lamp having an outer jacket disposed in surrounding relation to the arc tube also provided a more practical solution to the issue of mounting the lamp within a lighting fixture and moreover, provided such an electrodeless HID lamp with a thermal management characteristic that did not interpose additional elements between the arc tube and the coil that could otherwise interfere with the excitation of the gas within the arc tube. An example of an electrodeless HID lamp having an efficient coupling between the arc tube and the coil and also illustrating the use of an outer jacket to avoid some of the previously discussed problems experienced by bare arc tubes, can be found in U.S. Pat. No. 4,871,946 issued to Witting et al. on Oct. 3, 1989 and assigned to the same assignee as the present invention. In this patent, it can be seen that the coil is disposed within the outer jacket thereby requiring that, as between the coil leads and the outer jacket, there must be a metal to glass sealing arrangement to insure the integrity of the inner portion of the outer jacket. It would be advantageous if an electrodeless HID lamp having an outer jacket could be provided wherein the need for a metal to glass seal could be avoided altogether.
Other examples of electrodeless HID lamps can be found in U.S. Pat. Nos. 4,894,589 and 4,894,590 which issued on Jan. 16, 1990 and are assigned to the same assignee as the present invention. It will be noted that these inventions are primarily directed to the manner in which the plasma arc discharge is started by means of introducing an initial high voltage pulse over a pair of starting electrodes disposed in close proximity to the coil and the arc tube. Both of these patents illustrate the use of a bare arc tube and there is no discussion as to whether the starting techniques of these inventions could be applied to an electrodeless HID lamp having an outer jacket surrounding the arc tube. An example of an invention which could be used in such a situation can be found in
U.S. patent application Ser. No. 07/622,026 filed on Dec. 4, 1990 for a Discharge Starting Aid for an Inductively Coupled Arctube and assigned to the same assignee as the present invention. This application discusses an arrangement for starting the electrodeless HID lamp without the need for electrodes. It would be further advantageous if an electrodeless HID lamp having an outer jacket could accommodate the use of an electrodeless starting aid and still provide for the ability to mount the lamp in a practical lighting fixture and achieve optimum thermal performance characteristics without interference between the arc tube and the excitation coil.